Apparatus, systems and methods for measuring flow and pressure within a vessel

ABSTRACT

Exemplary embodiments of apparatus, system and method can be provided to measure a flow of fluid within an anatomical structure. For example, it is possible to use at least one first probe arrangement structured to be insertable into a vessel and configured to direct at least one radiation to at least one portion of the anatomical structure. Further, it is possible to provide at least one second arrangement which configured to detect an interference between a first radiation provided from the fluid via the probe arrangement and second a second radiation provided from a reference path as a function of wavelength thereof. Further, at least one third arrangement can be provided which is configured to determine at least one characteristic of the fluid as a function of the interference.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is based upon and claims the benefit of priority from U.S. Patent Application Ser. No. 61/225,418, filed on Jul. 14, 2009, the entire disclosure of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The invention was made with the U.S. Government support under Grant No. HL076398 awarded by the National Institutes of Health. Thus, the U.S. Government has certain rights in this invention.

FIELD OF THE DISCLOSURE

Exemplary embodiments of the present disclosure relates generally to measuring apparatus, systems and methods, and more particularly to apparatus, systems and methods for measuring flow and pressure within a vessel.

BACKGROUND INFORMATION

Interventional cardiologists measure intracoronary blood flow using an ultrasound Doppler guide wire to evaluate coronary vascular function and guide percutaneous coronary intervention (PCI). However, Doppler ultrasound requires a separate, standalone instrument and is difficult to use because the Doppler signal is dependent on the orientation of the transducer's tip. Optical Coherence Tomography (“OCT”), including Fourier Domain OCT (including but not limited to Optical Frequency Domain Imaging (OFDI), Swept Source Optical Coherence Tomography (SS-OCT), and Spectral-Domain Optical Coherence Tomography (SD-OCT)—some of which, are described in described in International Patent Application PCT/US2004/029148, filed Sep. 8, 2004 which published as International Patent Publication No. WO 2005/047813 on May 26, 2005, U.S. patent application Ser. No. 11/266,779, filed Nov. 2, 2005 which published as U.S. Patent Publication No. 2006/0093276 on May 4, 2006, and U.S. patent application Ser. No. 10/501,276, filed Jul. 9, 2004 which published as U.S. Patent Publication No. 2005/0018201 on Jan. 27, 2005, and U.S. Patent Publication No. 2002/0122246, published on May 9, 2002) use low coherence interferometry or optical frequency domain interferometry to measure scattering as a function of depth. OCT and its derivatives are fiber-optic catheter-based imaging techniques that use low coherence interferometry to visualize coronary artery microstructure.

If intracoronary blood flow could be measured using an OCT system and catheter, then imaging and functional perfusion measurements could be conducted easily, rapidly, and without additional cost above and beyond that of an OCT device and catheter.

Previously, another concept of utilizing an intravascular OCT device to measure flow within a vessel has been described—see X. Li et al., “Intraluminal fiber-optic Doppler imaging catheter for structural and functional optical coherence tomography,” Opt Lett, 26(23), 1906-8 (2001). This procedure described in such publication, however, utilized phase processing of the OCT signal to determine the Doppler shift of the returned light. This prior approach was limited by the directionality factors that can also limit ultrasound, and furthermore is more difficult to implement in the OCT systems.

Thus, it may be beneficial to address and/or overcome at least some of the deficiencies of the prior approaches, procedures and/or systems that have been described herein above.

OBJECTS AND SUMMARY OF EXEMPLARY EMBODIMENTS OF THE DISCLOSURE

It is therefore one of the objects of the present invention to reduce or address the deficiencies and/or limitations of such prior art approaches, procedures and systems.

It is another object according to a further exemplary embodiment of the present disclosure to provide an OCT catheter/device/apparatus that can be structured or configured to obtain flow information distally and/or proximally to the imaging window. According to still another object an additional exemplary embodiment of the present disclosure, it is possible to provide a guide wire that is capable of measuring flow at one or more locations within the artery simultaneously. Yet another object of a further exemplary embodiment of the present disclosure is to provide an arrangement, system, apparatus and/or method to convert flow information into sound. It is a further object of a further exemplary embodiment of the present disclosure to determine pressure within the coronary artery using flow and structural information obtained by OCT or another structural imaging modality such as IVUS.

According to yet another object of a further exemplary embodiment of the present disclosure, information can be obtained or determined regarding a property of the fluid within the vessel by, e.g., extrapolating measurements obtained from another portion of the fluid in the vessel. It is a still further object of a still further exemplary embodiment of the present disclosure to convert the OCT-derived flow information into an audio waveform. In addition, it is another object of an additional exemplary embodiment of the present disclosure to convert the OCT flow and pressure information into derived clinical parameters coronary flow reserve (CFR), Fractional Flow Reserve (FFR), Average Peak Velocity (APV) and other metrics known to those having ordinary skill the art.

Further, according to one exemplary embodiment of the present disclosure, an exemplary correlation procedure, system and apparatus can be provided that can have, e.g., reduced or minimal directionality limitations and can provide can be implemented (e.g., either partially or entirely) via a software arrangement or a software program.

For example, in a particular exemplary embodiment of the present disclosure, it is possible to utilize an existing OCT device, system and/or apparatus that can be configured for, e.g., an insertion in the coronary artery to obtain information regarding a flow of blood within the artery.

Accordingly, exemplary embodiments of apparatus, system and method can be provided to measure a flow of fluid within an anatomical structure. For example, it is possible to use at least one first probe arrangement structured to be insertable into a vessel and configured to direct at least one radiation to at least one portion of the anatomical structure. Further, it is possible to provide at least one second arrangement which configured to detect an interference between a first radiation provided from the fluid via the probe arrangement and second a second radiation provided from a reference path as a function of wavelength thereof. Further, at least one third arrangement can be provided which is configured to determine at least one characteristic of the fluid as a function of the interference. The third arrangement can determine the characteristic(s) as a function of an intensity of the interference.

For example, the characteristic(s) can comprise particular parameters which includes flow, viscosity, density, velocity, coronary flow reserve, fractional flow reserve, coronary flow velocity reserve, average peak velocity, maximum peak velocity, average velocity and/or pressure of the fluid within the vessel. The characteristic(s) can comprise a multi-dimensional distribution of the particular parameters. The third arrangement can determine the characteristic(s) at multiple longitudinal locations within the vessel. The third arrangement(s) can determine the characteristic(s) at least one proximal or distal to stenosis or blockage. The third arrangement(s) can be further configured to determine the property and/or a geometry of a wall of the vessel. The property of the wall can be a luminal contour or a bio-mechanical property of the wall, or a tissue characteristic of the wall. The third arrangement(s) can determine at least one characteristic of a wall of the vessel. The first probe arrangement(s) can include a catheter, a wire and/or a sheath. The fluid can comprise blood, transparent medium, and/or a combination thereof. The fluid can comprise \blood, transparent medium, and/or a combination thereof. A wavelength of at least one of the first radiation or the second radiation can vary over time. The second arrangement can include at least one array of detectors, each configured to detect a separate wavelength band of the interference. The third arrangement can determine the characteristic using a correlation procedure. The third arrangement can determine the as a function of a distance of the fluid being measured from the at least one first probe arrangement.

According to another exemplary embodiment, the third arrangement can be further configured to extrapolate further characteristics of the fluid where the fluid is not measured based on the at least one characteristic and information regarding a property of a wall of the vessel. The characteristic can be a pressure of the fluid within the vessel, and the third arrangement can determine the pressure as function of a property of a wall of the vessel. The third arrangement can determine the characteristic by analyzing a speckle pattern of an image associated with the fluid. The third arrangement can be further configured to determine at least one three-dimensional information of a wall of the vessel using the interference. The first probe arrangement can be configured to be immobile during operation of the apparatus. The third arrangement can determine the characteristic as a function of time, and/or determine synchronously with a further physiological measurement.

The further physiological measurement can be an EKG, heart rate, systolic or diastolic blood pressure, maximal flow, minimal flow, arterial pressure and/or a pressure measurement. The third arrangement can determine the characteristic before and/or after an administration of a pharmacologic agent. The first probe arrangement can direct the radiation to the portion along an axis which is approximately perpendicular to the direction of extension of the at least one first probe arrangement. The third arrangement can generate a sound based on the characteristic. A position of the first probe arrangement can be based on the sound. The third arrangement can generate the fractional flow reserve based on a pressure within the vessel. An additional arrangement can be provided for measuring pressure. The further arrangement can be configured to generate pressure information based on electromagnetic radiation transmitted through the catheter. The further arrangement can comprise a Fabry-Perot and/or or a fiber grating sensor. It is a further object of this invention to guide the decision to perform PCI and to also assess the success of PCI (in a manner analogous to FFR).

In one embodiment the invention provides an apparatus to measure a flow of fluid within an anatomical structure, including: at least one first probe arrangement structured to be insertable into a vessel and configured to direct at least one radiation to at least one portion of the anatomical structure; at least one second arrangement which configured to detect an interference between a first radiation provided from the fluid via the probe arrangement and second a second radiation provided from a reference path as a function of wavelength thereof; and at least one third arrangement which is configured to determine at least one characteristic of the fluid as a function of the interference. The at least one third arrangement may determine the at least one characteristic at multiple longitudinal locations within the vessel. The at least one third arrangement may determine the at least one characteristic at least one proximal or distal to stenosis, blockage or stented segment.

These and other objects, features and advantages of the exemplary embodiment of the present disclosure will become apparent upon reading the following detailed description of the exemplary embodiments of the present disclosure, when taken in conjunction with the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present disclosure will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the present disclosure, in which:

FIG. 1A is a schematic diagram of an exemplary embodiment of an OCT catheter apparatus, system and/or arrangement according to the present disclosure;

FIG. 1B is an exemplary cross-sectional image obtained using the exemplary OCT catheter apparatus shown in FIG. 1A;

FIG. 2 is a set of side views of an image generated using OFDI data obtained from the coronary artery of a living swine according to an exemplary embodiment of the present disclosure;

FIG. 3 is an exemplary image representing flow as a function of distance from an exemplary OFDI catheter and as a function of time, obtained from the data in FIG. 2B according to an exemplary processing method/procedure of the present disclosure;

FIG. 4A is an exemplary image graph representing a flow distribution as a function of time, obtained from the data associated with the exemplary shown in FIG. 3 according to the exemplary processing method of the present disclosure (S=systole, D=diastole). Figure

FIG. 4B is an exemplary image of an audio trace obtained from the flow distribution as a function of time associated with the graph of FIG. 4A according to an exemplary embodiment of the present disclosure;

FIG. 5 is an exemplary graph representing a peak flow velocity derived from an exemplary extrapolation of the flow velocity information derived from measured data near to the catheter apparatus/device/system according to an exemplary embodiment of the present disclosure;

FIG. 6A is a side view of a schematic diagram of an exemplary embodiment of a system and/or arrangement according to the present disclosure that includes an additional apparatus for measuring intraluminal pressures; and

FIG. 6B is an enlarged side view of the schematic of the system and/or arrangement of FIG. 6B.

Throughout the figures, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the subject disclosure will now be described in detail with reference to the figures, it is done so in connection with the illustrative embodiments. It is intended that changes and modifications can be made to the described exemplary embodiments without departing from the true scope and spirit of the subject disclosure as defined by the appended claims.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

A schematic diagram of an exemplary embodiment of an OCT catheter, system and/or arrangement is shown in FIG. 1A. For example, FIG. 1A illustrates such exemplary OCT catheter, system and/or arrangement which includes certain exemplary mechanical and/or optical elements that can be utilized in various exemplary catheter designs and/or configuration. In particular, as shown in FIG. 1A, the exemplary OCT catheter, system and/or arrangement can include an inner core 100, which can contain at least one fiber optic arrangement 75 (e.g., an optical fiber configuration which can include one or more fibers) that is coupled to an OCT system, apparatus or arrangement 50 which can include a processor and a storage medium (e.g., hard drive, CD-ROM, floppy disk, memory stick, combination thereof, etc) at a proximal end thereof (e.g., via an optical rotary junction 100), and can focus and redirect the light at a distal end 110 thereof.

As shown in FIG. 1A, at least one inner core or cable 105 can rotate and/or translate along any direction (e.g., along a direction of the arrangement 75) to provide one- or two-dimensional scanning via distal optics 115 by an optical beam 110 directed to an anatomical sample or structure. An inner core of the cable 105 can be enclosed in a transparent sheath 120 that can accommodate a guidewire. The inner core or cable 105 and/or the entire exemplary OCT catheter, system and/or arrangement can rotate and or translate laterally, and provide exemplary structural information in the form of an exemplary two-dimensional image 130 (shown as a cross-sectional image in FIG. 1B) or a three-dimensional image from the artery wall, which can be acquired in the presence of a saline or radio-contrast flush to remove blood from the field of view or dilute the blood to obtain clear images of the artery wall.

One exemplary embodiment of the present disclosure can provide an exemplary system and/or method is to utilize and existing OCT catheter to obtain flow information. In the preferred embodiments, blood or some other scattering substance is allowed to flow freely through the artery. In the preferred embodiment, the OCT catheter is not controlled to move and the optics remain stationary within the sheath. The OCT catheter's optics are also preferably oriented so that it is measuring signal from the at least one of fluid within the lumen of the vessel and the vessel wall. Motion of the scatterers can be measured using the OCT catheter, providing a record of the scatterer motion as a function of depth and time (FIG. 2). FIG. 2 depicts such two an exemplary image providing dimensional information where x axis 205 represents time and the y axis 207 represents distance from catheter. Because the scatterer has a limited width e.g., the size of the red blood cell is approximately 8 μm, if the scatterer moves rapidly 210 and 240, the signal returned by the scatterer remains for only a relatively small time 210, whereas if the scatterer moves slowly the signal returned by the scatterer remains for a relatively long time 220 and 230. This information can be processed according to the exemplary correlation methods, such as at least one of spatial and temporal autocorrelation, of the present disclosure to obtain flow information as a function of distance from the catheter optics and also as a function of time.

Described below are exemplary embodiments of a processing method for obtaining flow information from the OCT data according to the present invention. In this exemplary embodiment according to the present disclosure can be utilize which can use am exemplary speckle decorrelation method to measure flow because, unlike Doppler, it is relatively insensitive to catheter positioning and does not require modification of OCT system hardware or catheters. In this exemplary embodiment, Windowed (2.5 msec) autocorrelations along time can be computed for each depth location and for all times. The first zero crossing beyond the main autocorrelation peak is identified. The time constant (exponential fit), which has a relationship to flow velocity, can be computed from the autocorrelation peak (up to the first zero crossing) at each time and depth point. Alternatively, the flow information can be obtained by determining the width of the peak or height of the peak. In another embodiment, the autocorrelation can be fit using a multi-exponential, polynomial, Gaussian, or Lorentzian function or another function known for peak fitting in the art. Flow information (time constant reciprocal) is plotted in a 2D image 300 as a function of depth 320 and time 310 where the intensity of the image is the reciprocal of the time constant 330. Exemplary flow distributions (time constant PDFs) were then computed for each time point to provide an image 400 corresponding to the flow information as a function of at least one of time and depth 400 (see, e.g., FIG. 4A). In FIG. 4A, the flow distributions 400 can also be splotted adjacent to the EKG 410 that was acquired simultaneously with the OCT information. Alternatively the flow information can be obtained and plotted at least partially synchronously with exemplary pressure information. An ultrasound Doppler flow velocity distribution image 405 is also shown for comparison. In one embodiment, the flow distribution is converted into an audio waveform 420 as shown in FIG. 4B by providing the PDF's in the frequency space, transformed to audio frequencies, and performing an inverse Fourier transform. The audio waveform may be of use in determining that the catheter optics are positioned correctly and pointed in a direction towards the lumen of the artery and not towards the artery wall. The audio waveform may also be utilized to confirm an adequate flow signal.

In another embodiment, one or more additional arrangements can be provided within the transparent sheath of the OCT catheter. The exemplary arrangement can contain an optical waveguide that is designed to measure LCI or OFDR (also known as M-mode OFDI data) data proximal to, distal to, or within the imaging window of the OCT catheter. In this manner, the flow information can be obtained far distal for instance from the imaging window, which may be advantageous for obtaining flow within a reference segment. In another embodiment, the optical waveguide can be provided within a guide wire and operates with or without OCT imaging. In yet another embodiment, the processing method includes methods for obtaining structural information and also flow information and combining this information using theoretical relationships between flow, structure, and pressure to determine the pressure within the vessel. In another embodiment, the peak flow envelope is determined from the PDF's. In yet another embodiment, flow information is obtained as a function of longitudinal position along the vessel, at several different positions in relation to a stenosis, in a reference vessel that is less diseased than the interrogation site, or in conjunction with the administration of a pharmacologic agent to induce hyperemia. In another exemplary embodiment the flow information is converted into other measurements such as coronary flow reserve (CFR), coronary flow velocity reserve (CVR) or relative CVR (rCVR) using methods known in the art. In yet a further embodiment pressure information is converted into Fractional Flow Reserve (FFR) information using methods known in the art.

Because the penetration depth of light through blood may not allow the measurement of depth dependent flow information at all locations within the vessel, a further embodiment includes an apparatus that at least partially dilutes the blood so that the entire flow distribution or the maximal flow velocity within the vessel can be measured.

In yet another embodiment, the coronary flow is derived from the flow velocity information using knowledge of the geometrical configuration of the vessel. In yet another embodiment, the flow velocity information can be acquired proximally to the catheter, within the penetration depth of the OFDI or SD-OCT light and structural information regarding the artery wall is also obtained using intravascular imaging with the OFDI or SD-OCT light or via another imaging means such as angiography, CT, or IVUS known in the art. In an exemplary combination with the regional flow information, the structural information can be used to extrapolate flow information around at least one of other locations in the vessel or as a composite of the flow profile across the entire vessel (i.e. average flow, peak flow) using flow equations or fluid dynamic modeling equations such as the Navier-Stokes equations. An exemplary example of such a measurement is shown in the exemplary graph of FIG. 5, where average flow was derived from flow velocity information calculated from M-mode OFDI data obtained near to the catheter. The flow velocity information was fit to Navier-Stokes equation using artery geometry information including the luminal contour dimensions that were obtained via OFDI measurement of luminal diameter characteristics of the vessel phantom. The flow velocity information was used to extrapolate flow velocities elsewhere within the vessel phantom structure. These values derived from the experiment and calculations 510 can be converted into absolute flow rates using assumptions of laminar flow and compared to known flow rates 520, demonstrating a linear relationship with a high degree of correlation 530.

In another exemplary embodiment of the present disclosure, the vessel geometry information may be measured as a function of time to obtain biomechanical information such as shear stress using geometrical methods known in the art and the flow velocity information may be measured as a function of at least one of biomechanical information, spatial location, and time. In yet another embodiment, the flow information may be used in conjunction with geometrical information about the vessel wall to derive intraluminal pressure measurements. In yet another embodiment, shown in FIG. 6 A, the exemplary OFDI catheter 600, used to obtain information from the artery wall 610 can contain an additional apparatus 620 that facilitates the measurement of intraluminal pressure. This exemplary apparatus may contain an optical fiber 621 pressure sensor 623 that obtains pressure 626 measurements by optical means such as by detecting the interference between the front 627 and back 624 faces of a diaphragm 625, which is affected by pressure 626 and alters the distance between the faces of the diaphragm 628, such as a -based Fabry Perot sensor or the like. In yet another embodiment, this optical sensor utilizes the same or similar source of electromagnetic radiation as the imaging or flow velocity measurement electromagnetic radiation.

The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. Indeed, the arrangements, systems and methods according to the exemplary embodiments of the present disclosure can be used with and/or implement any OCT system, OFDI system, SD-OCT system or other imaging systems, and for example with those described in International Patent Application PCT/US2004/029148, filed Sep. 8, 2004 which published as International Patent Publication No. WO 2005/047813 on May 26, 2005, U.S. patent application Ser. No. 11/266,779, filed Nov. 2, 2005 which published as U.S. Patent Publication No. 2006/0093276 on May 4, 2006, and U.S. patent application Ser. No. 10/501,276, filed Jul. 9, 2004 which published as U.S. Patent Publication No. 2005/0018201 on Jan. 27, 2005, and U.S. Patent Publication No. 2002/0122246, published on May 9, 2002, the disclosures of which are incorporated by reference herein in their entireties. It will thus be appreciated that those skilled in the art will be able to devise numerous systems, arrangements and methods which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the present disclosure. In addition, to the extent that the prior art knowledge has not been explicitly incorporated by reference herein above, it is explicitly being incorporated herein in its entirety. All publications referenced herein above are incorporated herein by reference in their entireties. 

What is claimed is:
 1. An apparatus to determine at least one characteristic of a flowing fluid within an anatomical structure, comprising: at least one probe coupled to a radiation source, the at least one probe comprising distal optics and being structured to be insertable into a vessel, the at least one probe configured to: direct, using the distal optics, at least one radiation from the radiation source to at least one portion of the anatomical structure at a longitudinal position along the anatomical structure, and receive, using the distal optics, a first radiation from the flowing fluid within the anatomical structure at the longitudinal position based on directing the at least one radiation from the radiation source to the at least one portion of the anatomical structure, the at least one probe and the distal optics being configured to remain stationary during operation of the apparatus such that the distal optics continuously direct the at least one radiation toward the at least one portion of the anatomical structure and receive returned radiation based on the directed at least one radiation; a reference path coupled to the radiation source and to a reference structure, the at least one radiation from the radiation source being directed to the reference structure via the reference path, a second radiation being received from the reference structure via the reference path based on the at least one radiation from the radiation source being directed to the reference structure; at least one detector to detect an interference signal between the first radiation provided from the flowing fluid via the at least one probe and the second radiation provided from the reference structure via the reference path as a function of at least one wavelength of at least one of the first radiation or the second radiation, the interference signal identifying a scattering substance at a plurality of depths within the flowing fluid at the longitudinal position; and a processor configured to determine, based on data obtained while the at least one probe and the distal optics remain stationary, the at least one characteristic of the flowing fluid using speckle decorrelation analysis of the scattering substance at the plurality of depths within the flowing fluid based on an intensity change in the interference signal detected from the flowing fluid at at least two different points in time.
 2. The apparatus according to claim 1, wherein the at least one characteristic comprises particular parameters which include at least one of: viscosity, coronary flow reserve, fractional flow reserve, coronary flow velocity reserve, average peak velocity, maximum peak velocity, or peak velocity of the flowing fluid within the vessel.
 3. The apparatus according to claim 2, wherein the at least one characteristic comprises a multi-dimensional distribution of the particular parameters.
 4. The apparatus according to claim 1, wherein the processor is configured to determine the at least one characteristic of a wall of the vessel.
 5. The apparatus according to claim 1, wherein the at least one probe includes at least one of: a catheter, a wire, or a sheath.
 6. The apparatus according to claim 1, wherein the flowing fluid comprises at least one of: blood, transparent medium, or a combination thereof.
 7. The apparatus according to claim 1, wherein a wavelength of at least one of the first radiation or the second radiation varies over time.
 8. The apparatus according to claim 1, wherein the at least one detector comprises at least one array of detectors, each configured to detect a separate wavelength band of the interference signal.
 9. The apparatus according to claim 1, wherein the processor is configured to determine a distance between the flowing fluid and the at least one probe to determine the at least one characteristic.
 10. The apparatus according to claim 1, wherein the processor is further configured to extrapolate further characteristics of the flowing fluid where the flowing fluid is not measured based on the at least one characteristic and information regarding a property of a wall of the vessel.
 11. The apparatus according to claim 1, wherein the at least one characteristic is a pressure of the flowing fluid within the vessel, and wherein the processor is configured to determine the pressure as function of a property of a wall of the vessel.
 12. The apparatus according to claim 1, wherein the processor is configured to determine the at least one characteristic synchronously with a further physiological measurement.
 13. The apparatus according to claim 12, wherein the further physiological measurement is at least one of: an EKG, a heart rate, a systolic blood pressure, a diastolic blood pressure, a maximal flow, a minimal flow, an arterial pressure, or a pressure measurement.
 14. The apparatus according to claim 1, wherein the at least one probe is configured to direct the at least one radiation to the at least one portion along an axis which is approximately perpendicular to the direction of extension of the at least one probe.
 15. The apparatus according to claim 1, wherein the processor is configured to generate an audio waveform based on the at least one characteristic.
 16. The apparatus according to claim 1, wherein the processor is configured to generate a fractional flow reserve based on a pressure within the vessel.
 17. The apparatus according to claim 1, further comprising a pressure measurement device for measuring pressure.
 18. The apparatus according to claim 17, wherein the pressure measurement device is configured to generate information regarding the pressure based on the at least one electromagnetic radiation transmitted through the at least one probe which includes a catheter.
 19. The apparatus according to claim 17, wherein the pressure measurement device comprises at least one of: a Fabry-Perot or a fiber grating sensor.
 20. The apparatus according to claim 1, wherein the probe includes a beam steering structure which is configured to direct the at least one radiation to the at least one portion of the anatomical structure.
 21. The apparatus according to claim 20, wherein the beam steering structure includes at least one optical fiber configuration or the distal optics for transmitting the at least one radiation.
 22. The apparatus according to claim 20, wherein the radiation source comprises a light source providing the at least one radiation to the probe.
 23. The apparatus according to claim 22, wherein the light source provides the at least one radiation to the probe via at least one optical fiber configuration.
 24. The apparatus according to claim 1, wherein the processor, when determining the at least one characteristic of the flowing fluid using speckle decorrelation, is further configured to: identify, based on the intensity change in the interference signal detected from the flowing fluid, at least two time constants respectively associated with the at least two different points in time.
 25. The apparatus according to claim 1, wherein the processor, when determining the at least one characteristic of the flowing fluid using speckle decorrelation, is further configured to: determine, based on the intensity change in the interference signal detected from the flowing fluid, at least two autocorrelations respectively associated with the at least two different points in time, and identify, based on determining the at least two autocorrelations, at least two time constants respectively associated with the at least two different points in time.
 26. The apparatus according to claim 25, wherein the processor, when determining the at least one characteristic of the flowing fluid using speckle decorrelation, is further configured to: determine the at least one characteristic of the flowing fluid based on identifying the at least two time constants. 